Total cursors open, by session:
select a.value, s.username, s.sid, s.serial#
from v$sesstat a, v$statname b, v$session s
where a.statistic# = b.statistic# and s.sid=a.sid
and b.name="opened cursors current";
Source: http://www.orafaq.com/node/758
As far as I know queries on v$ views are based on pseudo-tables (“x$” tables) that point directly to the relevant portions of the SGA, so you can’t get more accurate than that; however this also means that it is point-in-time (i.e. dirty read).